1. Field of the Invention
The present invention relates to a PSA (pressure swing adsorption) apparatus for producing hydrogen gas for fuel cell and, more particularly, to a PSA apparatus improved so that such unnecessary gases such as CO, CH4, CO2, N2 and Ar which are by-products formed during the production of hydrogen gas to serve as a fuel cell energy source (fuel) can be efficiently removed by adsorption and the hydrogen gas production cost can be reduced accordingly.
2. Description of the Related Art
With measures for global warming prevention, it has become an important worldwide task in recent years to escape from the dependence of energy sources on crude oils and, not only in European advanced countries preceding in grappling with environmental preservation problems but also in the United States, Japan and other Asian countries, endeavors to put fuel cells using hydrogen gas as an energy source to practical use have been made actively.
While a number of studies are under way for developing improved methods of producing hydrogen gas to be used as a fuel for fuel cells, the production methods currently most practicable and most inexpensive use natural gas, LPG (liquefied petroleum gas), kerosene, gasoline, methanol or dimethyl ether as the raw material and produce hydrogen gas by reforming the same. In the processes for producing hydrogen gas by reforming such a raw material, for example in the process for producing hydrogen by reforming natural gas, the steam reforming technique is generally used most frequently. However, the steam reforming technique requires that the reforming reaction be caused to occur at a high temperature of about 700° C., and since this reforming reaction is an endothermic reaction, there arise problems: external heating becomes necessary, the reformer becomes increased in size and a prolonged period of time is required for starting. Autothermal reforming is a reforming technology improved about reformer size and starting time problems. According to this technology, the reaction involves not only a fuel and steam but is carried out in the presence of oxygen to combinedly utilize the heat generated by the oxidation reaction of the fuel and, accordingly, a reduction in size of the reformer and an improvement of the staring time are realized. In autothermal reforming, oxygen is sometimes used as an oxydant gas but, more conveniently, air is used. And air, natural gas and steam are mixed together and subjected to reforming, thereby a hydrogen-containing reformed gas is produced. Therefore, in this case, the reformed gas contains, together with hydrogen, at least steam (H2O), unreacted methane (CH4), carbon monoxide (CO), carbon dioxide (CO2) and nitrogen (N2). When air or oxygen-enriched gas manufactured by methods such as a PSA method from air as material is used as an oxydant gas, the reformed gas further contains argon (Ar) originating in air. Generally, the fuel hydrogen for fuel cell electric vehicles is required to have a hydrogen purity of about 4N (99.99% by volume (hereinafter “% by volume” is referred to as “%” for short)) or higher. And as for CO in particular, it is necessary to reduce the concentration thereof to a level of 10 ppm or lower from the viewpoint of preventing poisoning of platinum (Pt) used as an anode catalyst in solid polymer fuel cells. When the durability of fuel cells is taken into consideration, it is said to be necessary to reduce the CO concentration to a level of about 1 ppm or lower.
The hydrogen PSA method is a conventional method of purifying hydrogen gas from reformed gas. The hydrogen PSA method is a process of removing all of CO2, CH4, H2O, CO, N2 and Ar which are unnecessary gases in reformed gas, by using a plurality of adsorbents such as zeolite, activated carbon and alumina in combination under pressure swinging. In the case of the hydrogen gas which is to be fed to fuels cells for automobiles, it is also required that impurities other than CO be removed as well and, therefore, this hydrogen PSA method is generally employed in the case of producing fuel hydrogen by reforming a fossil fuel in hydrogen feeding stations.
When hydrogen gas is purified by the hydrogen PSA method, impurities other than H2 are removed by adsorption under elevated pressure to recover the product hydrogen gas. The adsorbent for PSA adsorbing the impurities CO, CH4, H2O, CO2, N2 and Ar is regenerated by release of the CO, CH4, H2O, CO2, N2 and Ar adsorbed thereon by an operation of reducing the adsorption tower inside pressure from the elevated level to ordinary pressure and an operation of purging with the product hydrogen gas. After regeneration of the adsorbent, the adsorption tower inside pressure is increased again. And the reformed gas flows into the tower to perform the product hydrogen gas purification operation.
In the conventional hydrogen PSA method, it is difficult to remove CO contained at a level of about 1% at a maximum in the hydrogen gas obtained by reforming a fossil fuel. And a large amount of an adsorbent (zeolite is generally used) is accordingly required, hence unfortunately the size of the hydrogen PSA equipment (adsorption tower size) becomes very large, and since the rate of recovery of the product hydrogen gas is low, the cost of hydrogen purification rises.
Various methods have so far been developed to cope with such problems. For example, Patent Document 1 discloses a method of increasing the hydrogen recovery rate by carrying out the step of the purging the adsorption tower after adsorption of impurities until at least a part of the purge gas introduced into the purging target tower is discharged from the tower, thereby the hydrogen gas recovery rate is increased from 70% (conventional method) to 76% at a maximum.
Patent Document 2 discloses a method of increasing the hydrogen gas recovery rate to 76% by utilizing, as a purge gas, the adsorption tower inside gas after finishing the adsorption step and increasing the amount of purge gas 2 to 7 times of the packed adsorbent volume. Further, Patent Document 3 discloses a method of reducing the hydrogen PSA equipment size and increasing the hydrogen recovery rate to 74% by using, as a single adsorbent, a zeolite having a faujasite structure with a Si/Al ratio of 1 to 1.5 and a lithium ion exchange rate of not lower than 95%.
However, these methods have limits in largely reducing the equipment size (adsorption tower size) since the CO gas adsorption capacity of the adsorbent is not sufficient. As regards the hydrogen recovery rate, improving measures by various methods such as those mentioned above have been investigated, however, any satisfactory measure is not available as yet.
Patent Document 4 discloses, as a method of reducing the adsorption tower size using a CO adsorbent, a method comprising disposing an adsorbent bed 2 obtained by successively layering an activated carbon layer 4, a CO adsorbent layer 5 and a zeolite layer 3 in the direction from the upstream side to the downstream side of the flow of a hydrogen-containing gas (cf. FIG. 9). From the explanation, since CO adsorbent is used with zeolite in this method, it is possible to reduce the amount of zeolite to be packed and thereby reduce the volume of the adsorption tower 1 and to reduce the amount of the product hydrogen gas to be used for adsorbent regeneration and thereby increase the hydrogen recovery rate. Further, since the CO adsorbent is higher in CO adsorption capacity as compared with zeolite when the CO concentration is high, this method is best suited for treating reformed gases containing CO at a level of not lower than 3 mol percent, in particular.
As a result of hydrogen purification tests using a CO adsorbent having a great CO adsorption capacity performed by the present inventors, it could be confirmed that the combined use of a CO adsorbent and zeolite can reduce the amount of zeolite to be packed. However, as described in Patent Document 4, it was also revealed that in the regenerating method of the adsorbent by feeding the high-purity hydrogen gas produced from an oxydant gas feeding route 6 to the adsorption tower 1 after pressure reduction to pass the gas through the adsorbent bed 2 thereby to release impurities adsorbed by the adsorbent bed 2, unfortunately the CO2 adsorbed mainly in the activated carbon layer 4 is released and then again adsorbed in the downstream-side zeolite layer 3 (zeolite being an adsorbent capable of adsorbing CO2 more strongly as compared with activated carbon), and CO2 re-adsorbed on zeolite cannot be regenerated (released) completely by the regeneration operation involving passage of high-purity hydrogen gas as a result. And it was also consequently revealed that the operation of removing impurity components in the oxydant gas by adsorption and the operation of regenerating the tower with high-purity hydrogen gas are repeated according to this method, the residual level of the re-adsorbed CO2 increases gradually at the time of reproduction in the zeolite layer 3 which initially has a sufficient level of CO adsorption capacity, thereby the CO adsorption capacity decreases, and as a result, the criterion “impurity CO concentration ≦1 ppm” required for hydrogen for fuel cells becomes no more satisfied.    [Patent Document 1]
JP-A-2002-177726    [Patent Document 2]
JP-A-2002-191923    [Patent Document 3]
JP-A-2002-191924    [Patent Document 4]
JP-A-2001-300244